Combined cycle power plants have recently been used from a viewpoint of more efficient use of energy. A combined cycle power plant includes a gas turbine plant, a steam turbine plant, a heat recovery steam generator, a condensate and feedwater system, and adopts a power generation method using the combination of a gas turbine and a steam turbine.
In the heat recovery steam generator and the condensate and feedwater system, pipes where condensate and feedwater flow are provided. For the long-term stable operation of the combined cycle power plant, it is required to retard the corrosion of inner surfaces of the pipes. One example of a presently adopted solution for retarding the corrosion of the inner surfaces of the pipes is water treatment of the condensate and the feedwater flowing in the pipes.
In a heat recovery steam generator of a conventional combined cycle power plant, a circulation boiler having a drum is typically used. Feedwater supplied to the circulation boiler is subjected to AVT (All Volatile Treatment) which supplies a deoxidant such as hydrazine and a pH adjusting agent such as ammonia to the water. The all volatile treatment is water treatment that forms a magnetite coating film on an inner surface of a pipe to protect the pipe from corrosion. Boiler water of the circulation boiler is subjected to phosphate treatment which supplies phosphate to the boiler water.
In a once-through boiler, water highly purified by, for example, a demineralizer is subjected to OT (Oxygen Treatment) which supplies a pH adjusting agent and a minute amount of oxygen to the water. The oxygen treatment is water treatment that forms a hematite coating film on an inner surface of a pipe to protect the pipe from corrosion.
FIG. 5 is a system diagram schematically illustrating a feedwater system 300 of a conventional combined cycle power plant. This illustrates an example where the all volatile treatment is performed in the feedwater system 300.
As illustrated in FIG. 5, the feedwater system 300 includes: a water quality adjusting device 347 which adjusts the water quality of condensate; a feed pump 341 which supplies feedwater to a heat recovery steam generator; an intermediate/low-pressure feedwater system feed pipe 343 and a high-pressure feedwater system feed pipe 344 where the feedwater discharged from the feed pump 341 flows; circulation boilers 370a, 380a, an intermediate-pressure economizer 370b, and a low-pressure economizer 380b which are supplied with the feedwater through the intermediate/low-pressure feedwater system feed pipe 343; and a circulation boiler 360a and a high-pressure economizer 360b which are supplied with the feedwater through the high-pressure feedwater system feed pipe 344. The circulation boiler 360a and the high-pressure economizer 360b are in a high-pressure feedwater system 360, the circulation boiler 370a and the intermediate-pressure economizer 370b are in an intermediate-pressure feedwater system 370, and the circulation boiler 380a and the low-pressure economizer 380b are in a low-pressure feedwater system 380. The high-pressure feedwater system 360, the intermediate-pressure feedwater system 370, and the low-pressure feedwater system 380 are disposed in the order mentioned along the flow of gas turbine exhaust flowing in the heat recovery steam generator toward a stack 350.
The turbine exhaust supplied from a steam turbine plant of the combined cycle power plant is condensed to condensate in a condenser 338. The condensate in the condenser 338 is supplied to the feed pump 341 through a gland condenser 340 by a condensate pump 339. During this period, a basic substance is supplied to the condensate from the water quality adjusting device 347. Then, the alkalized feedwater is supplied from the feed pump 341 to the high-pressure feedwater system 360 through the high-pressure feedwater system feed pipe 344, and to the intermediate-pressure feedwater system 370 and the low-pressure feedwater system 380 through the intermediate/low-pressure feedwater system feed pipe 343. Then, the feedwater is supplied to the high-pressure economizer 360b and the circulation boiler 360a which are in the high-pressure feedwater system 360, the intermediate-pressure economizer 370b and the circulation boiler 370a which are in the intermediate-pressure feedwater system 370, and the low-pressure economizer 380b and the circulation boiler 380a which are in the low-pressure feedwater system 380, and is phosphated in the circulation boilers 360a, 370a, 380a. 